Light-emitting diodes (LEDs) are well known in the prior art. A LED is formed by a semiconductor die, with a P-type semiconductor layer and an N-type semiconductor layer positioned on top of each other. A PN junction is defined between the P-type semiconductor layer and the N-type semiconductor layer. When a voltage is applied to the LED, holes in the P-type semiconductor layer and electrons in the N-type semiconductor layer are attracted and meet at the PN junction. When holes and electrons combine, photons are created, resulting in a radiation beam (light).
The LED may sit in a reflective cup that acts as a heat sink for transporting heat generated by the LED and a reflector for reflecting the created radiation beam.
LEDs typically emit a single wavelength of light, depending on the band-gap energy of the materials forming the PN junction. Nowadays, a variety of colors can be generated on the basis of the material used for making the LED. For instance, LEDs made with gallium arsenide produce infrared and red light. Other examples are gallium aluminum phosphide (GaAlP) for green light, gallium phosphide (GaP) for red, yellow and green light and zinc selenide (ZnSe) for blue light.
LEDs typically produce non-collimated radiation beams. Therefore, efforts have been made to collimate the light generated by a LED. Especially in the field of high-power LEDs, mixing of colors as well as beam-shaping and collimation optics are topics of frequent discussion. Even before the invention of LEDs, different ways of transforming a point source (in this case the LED) into a collimated radiation beam were known. An article entitled Le télescope de Newton et le télescope aplanétique, by M. Henri Chrétien, published in February 1922 in Revue Dóptique—Théorique et Instrumentale, describes the mathematics of transforming a point source into a collimated radiation beam using two reflective surfaces.
These mathematical techniques were used to develop optical elements to collimate a radiation beam generated by a LED. In this text, “collimated beam” is to be understood to denote radiation beams that are substantially parallel, i.e. parallel within 10° or 20°.
US 2004/0246606A1 describes such an optical element that is positioned over an optical source, such as a dome-packaged LED or an array of LEDs. The LED is positioned within a cavity of the optical element. The optical element is formed in such a way that the radiation beam generated by the LED enters the optical element via an entrance surface of the cavity. The radiation beam is reflected twice inside the optical device before it exits the optical element as a substantially collimated radiation beam. The optical element according to US 2004/0246606A1 will be explained in more detail below with reference to FIG. 1.
WO 2005/103562A2 addresses the problem of generating white light from a plurality of colored LEDs. According to this document, an optical manifold is provided for combining a plurality of LED outputs into a single, substantially homogeneous mixed output. Other known mixing techniques use mixing rods, light guides, reflectors or combinations thereof. However, these techniques are relatively large and bulky.